The invention is based on a delivery unit for delivering fuel from a supply tank to an internal combustion engine of a motor vehicle. German Offenlegungsschrift 40 20 521 discloses a delivery unit of this kind in which an electric drive motor drives an impeller to rotate which has radially outward pointing blades and which revolves in a pump chamber of a feed pump, which is embodied as a side channel pump, by means of which the fuel in a ring-shaped supply conduit in the axial face end chamber wall of the pump chamber in the region of the free ends of impeller blades is impelled into a rotating, whirling stream. This stream gets steadily stronger in the supply conduit from the low pressure fuel inlet opening to the end-of-delivery pressure outlet opening; the delivery pressure in the supply conduit also steadily increases via the conduit's circumference.
But the known fuel delivery units have the disadvantage that the efficiency of the pump decreases with increasing fuel temperature; in particular the delivery pressure at the outlet opening when the fuel is highly heated is far below the delivery pressure when the fuel is cold. This can be traced back to the whirling stream in the supply conduit, which causes a vacuum in its center, whose magnitude can reach 50% of the end-of-delivery pressure, so that highly heated fuel in this vacuum region begins to vaporize. The slower the pressure in the supply conduit increases, the more time the heated fuel has to vaporize; the vaporized fuel leads to a further delay of the pressure increase.
This effect is also involved in the fuel delivery units known from German Offenlegungsschrift 40 38 438, among other sources, whose feed pumps have a preliminary stage and a main stage; the preliminary stage is embodied by a side channel pump and the main stage by an internal gear pump, whose rotating pump parts are disposed on a common rotor shaft driven by an electric motor. In these known fuel delivery units, the highly heated fuel can lead to a failure of the preliminary stage, which strongly impairs the efficiency of the entire delivery unit and furthermore can cause cavitation damage, especially in the main stage.